1. Field of the Invention
This invention relates to a solid state microanemometer with improved structural integrity, sensing capability and versatility in application.
2. Brief Description of Related Art
A microanemometer is a device which measures the velocity and direction of fluid flow. This device is readily adaptable for use in a wide variety of applications. For instance, an array of these devices may be connected to associated computer interfaces to provide "intelligent" sensing of microflow patterns for medical, industrial or other applications.
Henderson, U.S. Pat. No. 4,930,347 discloses a solid state microanemometer micromachined out of a single deep level doped semiconductor crystal and includes four corner supports interconnected to four spanning members which form the resistor legs of a Wheatstone bridge. This device relies upon substantial thermal and electrical isolation of the resistor legs to provide high temperature sensitivity and fast response time.
For many applications, it is necessary for a microanemometer to withstand varying degrees of mechanical vibration over a long period of time. Otherwise, frequent breakage would necessitate excessive material and labor costs associated with replacement. Especially in sensing liquid flow, solid state microanemometers seem to be particularly susceptible to conditions of mechanical or physical wear.
Therefore, the success of a solid state microanemometer depends not only upon its high temperature sensitivity and fast response time, but also upon its ability to maintain these qualities under conditions of mechanical stress over a period of time.
It is also known that raising the operating temperature of a microanemometer increases its sensing capability. For a microanemometer of the type shown in U.S. Pat. No. 4,930,347, which utilizes self-heating resistors, this would suggest operation of the device at a higher current to increase the operating temperature of the resistors. However, because the self-heating resistors are interconnected by silicon, which has a high thermal conductivity, an increased current magnifies thermal shorting between the resistors, and thereby diminishes the capability of the device to detect small temperature differences. This capability may be for necessary for accurate sensing of fluid flow direction.
Henderson, U.S. Pat. No. 5,231,877 discloses a solid state microanemometer which is micromachined from a crystal to a shape with four thick external sides that define an outer rectangle, four thin sections that define an inner rectangle and four diagonally directed branches interconnecting the corners of the outer rectangle to the inner rectangle. Four semiconductor resistors located on the four inner sections form a sensing bridge. Each external side has a pair of electrical contacts that are electrically interconnected, via conductive leads that extend along the diagonal branches and partially along the inner sections, to one of the semiconductor resistors.
The '877 Henderson patent is an improvement over the '347 Henderson patent, as the thermal and electrical isolation of the resistors from each other permits higher operating temperatures and improved fluid flow sensing capability. However, the thin nitride bridging used to suspend the device has a tendency to break off when used in systems under mechanical stress.
Therefore there is a need for a microanemometer with a high temperature sensitivity that can be maintained under conditions of mechanical stress over a period of time.